<!DOCTYPE html>
<html>
<head>
  <style>
    body, html, iframe {
      width: 100%;
      height: 100%;
      margin: 0;
    }

    iframe {
      border: 0;
    }

    #space {
      width: 200px;
      height: 200px;
    }

  </style>
  <script>
    loaded = false;
    notifyBrowser = false;
    function notifyWhenLoaded() {
      if (loaded)
        window.domAutomationController.send("OUTER_LOADED");
      else
        window.notifyBrowser = true;
    }
    window.addEventListener('load', () => {
      window.loaded = true;
      if (window.notifyBrowser)
        notifyWhenLoaded();
    });
  </script>
</head>
<body>
  <div id="space"></div>
  <iframe src="input_box.html" >

 </body>
</html>